Eskom is implementing stage two rolling power cuts from 6pm to 10pm on Wednesday, it said on Twitter.
Earlier, the utility implemented stage one of rolling power cuts from 2pm to 10pm, but warned it could move to stage two.
This was due to a shortage of generation capacity as several units were currently out of service due to planned and unplanned outages.
The power system remained severely constrained and would be for the rest of the week, it said.
Eskom intended placing the units that were on unplanned maintenance back on line in order to increase available capacity.
Spokesman Khulu Phasiwe urged citizens to switch off all non-essential appliances.
Eskom had found that some municipalities left street lights on during the day and some people their house lights on.
During stage one blackouts 1000MW of the national load is cut, 2000MW during stage two, and 4000MW in stage three.